1. The Field of the Invention
The present invention relates to an optical glass, a method of producing the aforesaid optical glass, uses of the aforesaid optical glass, optical elements or preforms of such optical elements made with the aforesaid optical glass, and optical parts or optical components comprising such optical elements.
2. Related Art
Conventional optical glasses of the optical position claimed here (extreme hard flint position) generally contain PbO in order to achieve the desired optical properties, i.e. preferably a refractive index nd of 1.80≦nd≦1.95 and/or an Abbe number vd of 19≦vd≦28, but particularly the high refractive index. Hence, these glasses are chemically not very stable. As2O3 is furthermore often used as a fining agent.
Since the glass components PbO and As2O3 have been regarded as environmentally hazardous in recent years, most manufacturers of optical instruments and products tend to prefer the use of lead- and arsenic-free glasses. For use in high quality products, i.e. products of increased material grade, glasses with increased chemical stability are also constantly gaining in importance.
Known lead-free glasses of the hard flint or lanthanum hard flint position, with a high refractive index and a low Abbe number, generally contain large amounts of TiO2 in a silicate matrix, which leads to extreme crystallization instabilities and therefore glasses which are often not workable in a secondary hot pressing step, and which are very difficult to process mechanically due to high hardness.
Instead of the hitherto customary machining of optical components from glass in block or ingot form, production methods have recently gained in importance in which direct pressings, i.e. precision-moulded optical components and/or preforms for re-pressing which are as close as possible to the final contour, so-called “precision gobs”, can be obtained directly at the end of melting the glass. “Precision gobs” generally means preferably fully fire-polished, semifree- or free-formed glass portions, which can be obtained via various production methods.
For this reason the need for “short” glasses, i.e. for glasses the viscosity of which changes very strongly with temperature, has been reported more and more in the context of melting and moulding process technology. This method shows an advantage during processing, namely that it is possible to reduce the moulding times, and therefore the mould closure times, in precision moulding processes resulting in products close to final geometry. Hence, on the one hand the throughput is increased, and on the other hand the mould material is spared, which has a highly positive effect on the overall production costs. Furthermore, due to the faster solidification thereby obtained, it is also possible to work glasses with a stronger susceptibility to crystallization than in the case of correspondingly longer glasses, and pre-nucleation, which could be problematic in later secondary hot pressing, is avoided or at least drastically reduced.
For the same reason, there is likewise a need for glasses the temperature-viscosity profile of which in absolute terms comprises low temperatures in the moulding range. Through lower process temperatures, this also contributes to increased mould lifetimes and, through fast stress-free cooling, to low pre-nucleation rates. This also offers a greater range of potentially more cost-effective mould materials, which is significant particularly in precision moulding close to final geometry.
The prior art relevant to the invention is summarized in the following documents:
DE2905875Nippon KogakuEP1 468 974HoyaEP1 493 720HoyaJP09 188 540OharaEP1 382 582Ohara
According thereto, it is possible to produce glasses with a similar optical position or comparable chemical composition, although they show significant disadvantages in direct comparison with the glasses according to the invention:
The glasses described in the examples of DE 2905875 have an Nb2O5 content of equal to or less than 39 wt. %. The optical position desired for the glasses according to the invention therefore cannot be achieved without using large amounts of expensive, likewise high-index components in parallel with increased amounts of TiO2, albeit such that the crystallization stability of the glasses is critically reduced, i.e. the solubility limits in the Nb2O5—P2O5 matrix.
EP 1 468 974 (Prio '03) describes niobium phosphate glasses mandatorily containing bismuth. Owing to the intrinsic absorption of bismuth oxide, these glasses have poor transmission at the blue spectral edge. They are also more redox-sensitive compared to bismuth-free glasses, i.e. insufficiently oxidative melt control could lead to Bi0 colloids which cause a grey-violet colouration of the glasses. The process window for melting is thereby greatly reduced, which leads to increased production costs and potentially lower yields.
The glasses described in EP 1 493 720 (Prio '03) likewise derive from the niobium phosphate glass system, although they mandatorily contain either bismuth oxide (up to 37 wt. %) with the aforementioned disadvantages or large amounts of lithium oxide (up to 15 wt. %). Increased levels of lithium oxide lead to enhanced aggressivity of the melt in respect to the refractory material. Besides shorter equipment lifetimes, this leads to a strong ingress of the refractory material into the glass. In the case of platinum this leads to transmission losses at the blue spectral edge, and in the case of ceramic materials to enhanced susceptibility to crystallization in the melt as well as in case of primary and secondary hot pressing (for example re-pressing) by ingress of heterogeneous crystallization nuclei.
The glasses disclosed in JP 09 188 540 (Prio '95) have a maximum total alkaline-earth metal oxide content of 20 wt. %. This restricts the potential for adjusting a sufficiently “steep” viscosity-temperature profile and therefore the processability in moulding processes close to final geometry (for example “precision moulding”).
Despite the redox sensitivity of niobium phosphate glasses, the glasses described in EP 1 382 582 (Prio '02) have only extremely low levels of stabilizing antimony oxide (up to 0.03 wt. % maximum). This makes the melting process more susceptible to inevitable fluctuations, and increases the process costs because of increased monitoring work and potentially lower yields.